Conventional light engines for projection display devices, such as projectors and projection televisions, include a powerful light source for generating light, relay optics for transmitting the light, an image engine for converting the light into primary colors and selecting the appropriate color for each pixel, and projection optics for projecting the light onto a screen.
A Digital Light Processing (DLP®) system is illustrated in FIG. 1, and includes a light source 1 for focusing light 2 into a lightpipe 3, which ensures that the light 2 generated by the light source 1 is spatially uniform before entering the remainder of the system. Typically, the lightpipe 3 is comprised of a hollow pipe with a highly reflective inner surface or a solid piece of optically transparent material, e.g. a glass rod. The non-uniform light 2 enters the lightpipe 3 from the light source 1 and is reflected off of the walls thereof via total internal reflection (TIR). The light 2 mixes together as it propagates down the light pipe 3 forming a highly uniform source of light 4. The light source 1 and the lightpipe 3 are referred to as a polarization independent front-end unit. A color wheel 6 rotates to alternatively provide the three primary colors, i.e. blue, red and green, which are then focused onto a Digital Micromirror Device (DMD®) chip 7, via relay lens 8 and a TIR prism 9. The DMD chip 7 reflects the desired color combinations through a projection lens 10 onto a screen (not shown).
A Liquid Crystal on Silicon (LCOS) system is illustrated in FIG. 2, and includes a polarization dependent front end unit comprising a light source 11 for focusing light 12 into a polarization controlling lightpipe (PCLP) 13. The PCLP 13 not only ensures that the light is spatially uniform, but it also ensures that all of the input light has the same state of polarization, which is a requirement of the LCOS system. The light 12 enters a first polarization beam splitting (PBS) cube 14, which transmits p-polarized light directly to the PCLP 13, while reflecting s-polarized to a second PBS 16, which reflects the light through a polarization rotator 17, e.g. a ½-wave retarder. The polarization of the reflected s-polarized light is rotated by 90° to become p-polarized light, which then enters into the PCLP 13. Uniform light 18 exiting the PCLP 13 focused onto an LCOS image kernel 19 via a relay lens 20. The LCOS image kernel 19 includes dichroic filters for separating the light into primary colors, i.e. red, green and blue, and three LCOS panels (one for each primary color) for reflecting the desired combination of colors through a projection lens 21 onto a screen (not shown).
As light engine systems become less expensive and smaller, design requirements often demand that the optical path be folded before or after entering the lightpipe. Typically, several factors combine to create the folded path requirement, e.g. smaller packaging, light engine space envelope requirements, the specific light engine design, the orientation of the light engine or the operational requirements of the light source.
Unfortunately, light collection optics operate at a low focal ratio, e.g. approximately F/1, which causes the cone of light exiting the light source to converge rapidly as it approaches' the entrance to the lightpipe, thereby leaving little or no room to fold the light without causing a vignetting effect, resulting in a loss of luminous flux.
In conventional light engine systems the folding of light is conducted by flat mirrors positioned in the optical path. However, these folds are not typically positioned at the input or output ends of the lightpipe, because the high flux levels of the light at those points could crack or melt the flat mirrors or cause damage to the reflective coating thereon.
One attempt to solve the aforementioned problem includes inserting an elliptical reflector and a lens system for relaying the light from the light source to the lightpipe input. In this case a fold mirror could be positioned where the flux is not concentrated. However, this solution has increased costs, requires more space, and results in a decrease in lumens due to the addition of the lens system.
An object of the present invention is to overcome the shortcomings of the prior art by providing an efficient way to fold the converging light in a light engine between the light source and the lightpipe without lenses or flat mirrors.